1. Field of the Invention
This invention relates generally to an improved bonding method and more particularly to an improved vapor bonding method.
2. Description of the Prior Art
Soldering and reflow soldering methods for manufacturing electronic circuitry and the like are accomplished by the application of appropriate heat to the solder material and/or to the articles to be bonded together by soldering. Typically, the heat can be applied by using a soldering iron, by using a reflow oven, by using infrared radiation and more recently by placing the assembly to be soldered in an appropriate vapor atmosphere. Generally the soldering process includes the application of a flux to the surface to be soldered to clean the surface by the removal of any contaminate including any oxide layer developed thereon. The known methods for heating the surfaces to be soldered, while achieving the necessary objective of melting the solder, often, as an undesirable side effect, also necessarily subjects the flux to excessive heat which can result in a baked, polymerized product, often difficult to remove. This can be particularly critical when ultraclean circuitry and freedom from corrosive residue is desired. Furthermore, the application of the flux to the unit and the subsequent removal of the flux residues require additional steps, processing time and equipment.
Making solder bonds by the use of soldering irons, reflow ovens and infrared radiation are, of course, well known in the art. The idea of using a hot saturated vapor as a heat transfer medium to heat the article to be soldered to the desired temperature is of relatively recent origin. An example of such a technique is shown in U.S. Pat. No. 3,866,307, entitled "Method for Soldering, Fusing or Brazing," which patent describes the use of fluorocarbons, such as fluorinated polyoxypropylene, as heat transfer mediums which are heated to their boiling point in a suitable enclosure to produce a hot saturated vapor into which the article to be soldered is placed. Unfortunately, these liquids have relatively low solubility parameters and, as such, they are not particularly effective in cleaning the article to be soldered prior to the soldering operation, or in cleaning the article being soldered and removing a flux residue after the soldering operation.